Two cysteine-rich peptides, prokineticin 1 (PK1) and prokineticin 2 (PK2), have been identified. Prokineticins (PKs) are multi-functional peptides, which have been shown to stimulate gastrointestinal (GI) smooth-muscle contractions (Li et al., “Identification of two prokineticin cDNAs: Recombinant proteins potently contract gastrointestinal smooth muscle,” Mol Pharmacol 59:692-698 (2001)). PK1, also known as endocrine gland vascular endothelial growth factor (EG-VEGF), stimulates proliferation and migration of cells derived from endocrine glands, and promotes angiogenesis in the mouse ovary (LeCouter et al., “Identification of an angiogenic mitogen selective for endocrine gland endothelium,” Nature 412:877-884 (2001)). PK2, or mammalian Bv8, is believed to affect behavioral circadian rhythms in the suprachiasmatic nucleus (SCN) and promote angiogenesis in the testis (Cheng et al., “Prokineticin 2 transmits the behavioural circadian rhythm of the suprachiasmatic nucleus,” Nature 417:405-410 (2002); LeCouter et al., “The endocrine-gland-derived VEGF homologue Bv8 promotes angiogenesis in the testis: Localization of Bv8 receptors to endothelial cells,” Proc Natl Acad Sci USA 100:2685-2690 (2003)).
PK1 and PK2 are closely related and share significant sequence homology to mamba intestinal protein (MIT) (Schweitz et al., “Purification and pharmacological characterization of peptide toxin from the black mamba (Dendroaspis polylepis) venom,” Toxicon 28:847-856 (1990); Schweitz et al., “MIT(1), a black mamba toxin with a new and highly potent activity on intestinal contraction,” FEBS Lett 461:183-188 (1999)) and a frog skin secreted protein, Bv8. Bv8 is a potent stimulator of GI smooth-muscle contractions (Mollay et al. “Bv8, a small protein from frog skin and its homologue from snake venom induce hyperalgesia in rats,” Eur J Pharmacol 374:189-196 (1999)) and stimulates the sensitization of peripheral nociceptors (Negri et al., “Nociceptive sensitization by the secretory protein Bv8,” Br J Pharmacol 137:1147-1154 (2002)).
PKs bind and activate two closely related G-protein coupled receptors (GPCRs), prokineticin receptor 1 (PKR1) and 2 (PKR2), which are 87% identical by sequence (Lin et al., 2002, infra; Masuda et al., “Isolation and identification of EG-VEGF/prokineticins as cognate ligands for two orphan G-protein-coupled receptors,” Biochem Biophys Res Commun 293: 396-402 (2002); Soga et al., “Molecular cloning and characterization of prokineticin receptors,” Biochim. Biophys. Acta 1579: 173-179 (2002)). PKs stimulate Ca2+ mobilization in PK-receptor (PKR) expressing cells, presumably through receptor Gq protein interaction (Lin et al., “Identification and molecular characterization of two closely related G protein-coupled receptors activated by prokineticin/endocrine gland vascular endothelial growth factor,” J Biol Chem 277:19276-19280 (2002a)). Pertussis toxin (PTX) inhibits PK1-induced mitogen-activated protein kinase (MAPK) signaling (Lin et al., “Characterization of endocrine gland-derived vascular endothelial growth factor signaling in adrenal cortex capillary endothelial cells,” J Biol Chem 277:8724-8729 (2002b)), suggesting that PKRs may also couple to Gi proteins.
Sequence alignments have suggested that PKs have distinct N- and C-terminal domains (Bullock et al., “Structural determinants required for the bioactivities of prokineticins and identification of prokinectin receptor antagonists,” Mol. Pharmacology 65:582-588 (2004)). The N-terminal domain contains six amino acids (AVITGA) conserved among PKs from mammalian and nonmammalian species (id.). The C-terminal region contains ten conserved cysteines forming five pairs of disulfide bridges (id.). The pharmacological activity of a PK2 splice variant containing 21 extra amino acids inserted between exons 2 and 3 has also been studied (id.).